Qt4: accept close event on main interface (fix #4606)

If the event is ignored, the Qt4-X11 will reject the quit ICCM request.
This annoyingly caused VLC to inhibit logging out.
(cherry picked from commit 96fce709b0615071b5ddc17d8560a9c5794cc3b9)

Conflicts:

	modules/gui/qt4/main_interface.cpp

modules/gui/qt4/main_interface.cpp

@@ -1143,8 +1143,9 @@ void MainInterface::wheelEvent( QWheelEvent *e )
 void MainInterface::closeEvent( QCloseEvent *e )
 {
     //hide();
-    e->ignore();
     emit askToQuit();
+    /* Accept session quit. Otherwise we break the desktop mamager. */
+    e->accept();
 }
 
 void MainInterface::setInterfaceFullScreen( bool fs )